Rust-Stop Spray Strategies

Ohio researchers recently announced several concrete conclusions from studies conducted during 2005 and 2006 to determine what spray equipment will work best in a battle against Asian soybean rust (ASR). These studies are the most thorough and comprehensive of their kind to date, says Erdal Ozkan, Ohio State University (OSU) professor and Extension agricultural engineer.

“Our look at the performance of nozzles and spray equipment for both spray deposits and coverage is unique,” says Ozkan. “No other study used the different methods that we did to analyze spray equipment and nozzles together in one study.”

The six main conclusions from the spray studies, conducted by both OSU Extension and USDA Agricultural Research Service (ARS) personnel, are:

A medium spray quality (rather than fine or coarse) provides better droplet penetration into the plant canopy and better spray coverage when using conventional boom sprayers.

A short, low-density plant canopy helps to improve both spray coverage and spray deposits on lower and middle leaves, no matter which equipment delivers the spray.

A single flat-fan provides better spray coverage and spray deposition than cone or dual-fan nozzles in tall, dense plant canopy.

A mechanical canopy opener, or a rigid bar that bends the top of the plant canopy over ahead of spraying, helps improve spray penetration when using sprayers without air assistance.

In general, air-assisted technologies consistently performed best among tested products in delivering fungicide below the middle and lower-leaf canopy, where rust is most likely to develop, say the researchers. “The choice of spray equipment is crucial to soybean rust control, especially if the soybean canopy is dense and tall,” says Ozkan. “The most effective strategy under these conditions is to use an air-assisted sprayer.”

Nozzle selection was also important when using a conventional boom sprayer. Research conducted in 2005 clearly showed that a twin-flow pattern nozzle, such as the conventional TwinJet nozzle, did not perform as well as a single flat-fan nozzle in dense canopy. “If you use a conventional boom sprayer, then a flat-flan nozzle that provides a single-spray flow pattern will likely work better than a cone nozzle or a twin-pattern flat-fan nozzle,” says Ozkan, who worked with USDA-ARS scientists Richard Derksen and Heping Zhu to conduct the studies.

Air-assisted systems achieved equal performance with either a single flat fan or a cone nozzle, according to the research results. The studies also showed that droplet size is equally important for both conventional boom sprayers and air-assisted systems.

“Most product labels will likely recommend droplets in the fine to medium size, or in the 200-350 micron range for soybean rust control,” says Ozkan. “However, based on our studies, I would stick to the higher end of the fine and lower end of the medium range, or close to the 250-micron size.”

Research data from Brazil, where ASR infestations commonly occur, indicate that the most successful fungicide treatments are the ones that reach the plant's lower parts. “It's the older, lower leaves that are the most prone to rust infection,” says Anne Dorrance, OSU Extension plant pathologist.

In the U.S., the triazole or curative products will likely be the ASR fungicide of choice, particularly in northern states, says Dorrance. “They're what you'd use if your soybeans are in the later growth stage, with no other soybean diseases present,” she says. “That's because timing for the curative product is very narrow.”

However, if soybeans are early in the reproduction stage (R2-R3), the risk of rust is low and another soybean disease is present, such as frogeye leaf spot, then a strobilurin, or a protectant-type product would likely be a better choice, says Dorrance. “Both triazole and strobilurin fungicides are preventative products,” she emphasizes. “They are both applied in a preventative fashion.”

A successful spray application will not only include selecting the right fungicide product, but also the right spray equipment, adds Ozkan. For soybean growers without access to assisted spray technology, however, their success may depend on whether the soybean plant canopy is dense and tall or short and spread out, he says.

“Even if the canopy is relatively short and light, the air-assisted sprayer still works better than the conventional sprayer with a flat-fan nozzle,” he says. “However, in 2006 the performance was less noticeable than in 2005.”

ASR Spray Strategies Will Work For Aphids, Too

All the spray equipment tools that are most likely to perform well to control Asian soybean rust (ASR) will likely also work well to control soybean aphids, says Ron Hammond, an Ohio State University Extension entomologist with the Ohio Agricultural Research and Development Center (OARDC), Wooster, OH.

“To control aphids, the spray equipment needs to provide good penetration into the lower leaf canopy and deposit on the underside of leaves,” says Hammond. “To do that, you need a medium to small droplet size, which is different than when you are spraying herbicides, which typically require a larger-size droplet.”

However, timing is also crucial for optimum soybean aphid control. As a result, Hammond emphasizes the need for scouting prior to spraying.